This invention relates generally to a tire for use on a vehicle with suspension systems allowing little or no camber adjustment. More particularly, the invention relates to a racing tire with improved lateral force characteristics for use on solid rear axle race cars.
Laboratory studies have shown that a majority of a tire""s net cornering force is generated in the area of the shoulder on the inside of the direction of turn. Roll and lateral deflection to the outside tend to lift this inside shoulder, reducing the tire""s cornering capability. In many vehicles, especially high performance or racing vehicles, the vehicle""s suspension system compensates for the reduced cornering capability by cambering of the vehicle wheels. In cambering, the top of a wheel and tire are tilted toward the center of the radius of the turn for maximum cornering force.
For racing vehicles participating in NASCAR racing, such as in the Winston Cup and Busch Series""Grand National Division, cambering of the tire tilts the top of the tire and wheel inward toward the center of the oval track. The effect of such cambering on the contact patch of the tire as it travels about the track is such that on straight-aways the contact patch is eggshaped with the larger radius toward the inside of the track. This contact patch becomes more oval shaped while cornering, due to the effect of the roll and lateral deflection. This oval shaped contact patch is desirable as it enables a more even distribution of the forces across the tread during cornering, and provides for more stabile high speed cornering.
However, for NASCAR vehicles, the appropriate sanctioning bodies forbid or limit the cambering on the rear axle, thus there is no offsetting of the roll and lateral deflection during cornering. Due to the lack of camber, when the rear axle race tire undergoes significant lateral forces during high speed cornering the contact patch configuration becomes egg-shaped with the larger radius toward the outside of the turn. This results in a loss of tire contact patch area along the tire""s inside shoulder, relative to the turn, and reduced cornering stability for the vehicle.
The off-cambered half of a tire effectively requires less drop than the on-cambered side to support a given load. A tire""s shoulder drop and profile determine its relationship between load, camber, and footprint. Prior art tires designed to compensate for reduced camber preset have employed asymmetric shaped tires with unequal length sidewalls. The tires did not specifically alter the load/footprint relationship along the off-cambered shoulder.
U.S. Pat. No. 3,435,874 discloses varying the tread profile or the belt profile to achieve a desired offset of the force of structure and the force of conicity to result in a low total lateral slip force. The tread profile may be inclined at a single angle from one tread edge to the opposing tread edge, or differing angles.
U.S. Pat. No. 5,591,282 discloses a tire and vehicle system configured to compensate for the lack of camber on the rear axle of a vehicle. Therein, a support member is placed on the surface of the tire""s inboard sidewall. However, such a system adds weight to the tire, an undesirable result for a racing tire, where optimized weight reduction is required.
U.S. Pat. No. 5,620,538 discloses an asymmetrical racing tire for use on an oval race course. The tire is design for reduced belt edge separation on the outer sides of the tire, relative to the mounted wheel. The asymmetric tire has an outer diameter and thickness at the outer tread region greater than the outer diameter and thickness at the inner tread region. Again, such a tire has increased weight without compensating for the lack of camber on the rear axle of the vehicle.
European Patent 755 808A2 discloses a tire for a passenger vehicle wherein the tire has a sloped tread profile wherein the shoulder drops are unequal and the maximum tire height is offset from the tire centerline. When the tire is mounted on a vehicle, the shoulder with the greatest shoulder drop is to be mounted away from the vehicle body.
The present invention discloses a method of improving the lateral force characteristics of rear axle racing tires by eliminating the loss of the tire contact patch area along the tire""s inside shoulder, relative to the cornering of the vehicle, which occurs during chassis roll and lateral deflection.
In another aspect of the present invention, a tire to be set on the rear axle of a racing vehicle running on a circular race course, is disclosed. The profile of an outer tread region of the tread which is positioned to be further axially outward toward an outer side of a race course than a center line of the tine and a profile of an inner tread region of the tread portion which is positioned to be further axially inward toward an inner side of the race course than the center line of the tire are asymmetrical. The axially outer portion of the asymmetrical tread region of each tire to be mounted on the rear axle of the vehicle, relative to the race course, has a greater shoulder drop, where the shoulder drop is the difference between the maximum radial diameter of the tread measured on the outer tread contour and the diameter of the tread at the tread edge, than the axially inner portion of the asymmetrical tread region.
In further accordance with the disclosed invention, the ratio of the shoulder drop of the axially outer shoulder of each tire mounted on the rear axle of a noncambered vehicle to the shoulder drop of the axially inner shoulder of the ire is equal to 1.0 to 4.5.
In further accordance with the disclosed invention, the ratio of the shoulder drop of the axially outer shoulder of each tire mounted on the rear axle of a non-cambered vehicle to the shoulder drop of the axially inner shoulder of the tire is equal to 1.5 to 2.5.
Another disclosed invention is the method of mounting a tire on a rear axle of a racing vehicle for running on an oval race course. The profile of an outer tread region of tire tread which is position to be further axially outward toward an outer side of a race course than a center line of the tire and a profile of an inner tread region of the tire tread which is positioned to be further axially inward toward an inner side of the race course than the center line of the tire are asymmetrical. The tire is mounted wherein the shoulder drop, the difference between the maximum radial diameter of the tread and the radial diameter of the tread at the tread edge, of the axially outer portion of the tire, relative to the race course, has a greater shoulder drop than the axially inner portion of the tire.
In further accordance with the disclosed method of mounting a tire, the ratio of the axially outer shoulder drop to the axially inner shoulder drop is in the range of 1.0 to 4.5.
In further accordance with the disclosed method of mounting a tire, the ratio of the axially outer shoulder drop to the axially inner shoulder drop is in the range of 1.5 to 2.5.
Another disclosed invention is the method of molding an asymmetric tire by providing a tire, an asymmetrical mold, and molding the tire in the mold. The asymmetrical mold is characterized by a tread profile comprising axially opposing tread shoulder regions, each shoulder region having a maximum radial height of the mold tread profile, and a tread centerline having a minimum radial height of the mold tread profile. Each shoulder region is defined by a shoulder drop height equivalent to difference between the maximum radial height of the shoulder region and the minimum radial height of the tread mold profile at the tread centerline. The shoulder drop height of a first shoulder region is greater than the shoulder drop height of the opposing second shoulder region.
In further accordance with the disclosed method of molding an asymmetric tire the ratio of the shoulder drop of the first shoulder region to shoulder drop of the opposing second shoulder region is in the range of 1.0 to 4.5.
In further accordance with the disclosed method of molding an asymmetric tire the ratio of the shoulder drop of the first shoulder region to shoulder drop of the opposing second shoulder region is in the range of 1.5 to 2.5.
Definitions
xe2x80x9cApexxe2x80x9d means an elastomeric filler located radially above the bead core and between the plies and the turnup ply.
xe2x80x9cAsymmetric treadxe2x80x9d means a tread that has a tread configuration not symmetrical about the centerline CL or equatorial plane EP of the tire.
xe2x80x9cAxialxe2x80x9d and xe2x80x9caxiallyxe2x80x9d are used herein to refer to lines or directions that are parallel to the axis of rotation of the tire.
xe2x80x9cBeadxe2x80x9d means that part of the tire comprising an annular tensile member wrapped by ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes, toe guards and chafers, to fit the design rim.
xe2x80x9cBeltsxe2x80x9d refers to at least two annular layers or plies of parallel reinforcement cords having the same angle with reference to the equatorial plane of the tire as the parallel reinforcing cords in carcass plies.
xe2x80x9cCamberxe2x80x9d means the tilt of the front wheels of a vehicle; outward at top is positive.
xe2x80x9cCarcassxe2x80x9d means the tire structure apart from the belt structure, tread, undertread, and sidewall rubber over the plies, but including the beads.
xe2x80x9cCasingxe2x80x9d means the carcass, belt structure, beads, sidewalls, and all other components of the tire excepting the tread and undertread.
xe2x80x9cCircumferentialxe2x80x9d means lines or directions extending along the perimeter of the surface of the annular tire parallel to the Equatorial Plane (EP) and perpendicular to the axial direction.
xe2x80x9cEquatorial plane (EP)xe2x80x9d means the plane perpendicular to the tire""s axis of rotation and passing through the center of its tread.
xe2x80x9cGroovexe2x80x9d means an elongated void area in a tread that may extend circumferentially or laterally about the tread in a straight curved, or zigzag manner.
xe2x80x9cInner Sidexe2x80x9d of the disclosed invention means the side of the tire closest to the center of an oval race course when the tire is mounted on a vehicle located on the oval race course, and xe2x80x9cOuter Sidexe2x80x9d means the side of the tire closest to the outermost side of an oval race course when the tire is mounted on a vehicle located on the oval race course.
xe2x80x9cLateral Edgexe2x80x9d means the axially outermost edge of the tread as defined by a plane parallel to the equatorial plane and intersecting the outer ends of the axially outermost traction lugs at the radial height of the inner tread surface.
xe2x80x9cRadial tirexe2x80x9d means a belted or circumferentially-restricted pneumatic tire in which the ply cords which extend from bead to bead are laid at cord angles between 65xc2x0 and 90xc2x0 with respect to the equatorial plane of the tire.
xe2x80x9cSection heightxe2x80x9d means the radial distance from the nominal rim diameter to the outer diameter of the tire at its equatorial plane.
xe2x80x9cShoulderxe2x80x9d means the upper portion of sidewall just below the tread edge, effects cornering. Tread shoulder or shoulder rib means that portion of the tread near the shoulder
xe2x80x9cSidewallxe2x80x9d means that portion of a tire between the tread and the bead.
xe2x80x9cTreadxe2x80x9d means a molded rubber component which, when bonded to a tire casing, includes that portion of the tire that comes into contact with the road when the tire is normally inflated and under normal load.
xe2x80x9cTread Centerlinexe2x80x9d(CL) refers to the intersection of the equatorial plane (EP) with the tread.
xe2x80x9cTread Radiusxe2x80x9d is the radius or combination of radii describing the tread contour.
xe2x80x9cTread Arc Widthxe2x80x9d means the arc length of the tread surface in the axial direction, that is, in a plane parallel to the axis of rotation of the tire.